1. Field of the Invention
This invention relates to methods for the dewaxing of heavy hydrocarbon liquids to produce lube oil. More particularly, it relates to a method for the dewaxing of heavy hydrocarbon liquids, within a particular range of operating temperature and space velocity, by contact with a catalyst having a high alpha value.
2. Discussion of Prior Art
The dewaxing of hydrocarbons to liquids of lower pour point is a process of great commercial significance. Although alternatives exist, it is now evident that the use of shape-selective catalysts, such as a ZSM-5 type catalyst, to selectively convert those paraffins that contribute the most to high pour points has many advantages over other methods. Thus, catalytic dewaxing over shape-selective zeolites will likely be the most commercially significant dewaxing process in the hydrocarbon processing industry.
Catalytic dewaxing of hydrocarbon oils to reduce the temperature at which precipitation of waxy hydrocarbons occurs is a known process and is described, for example, in the Oil and Gas Journal, Jan. 6, 1975, pages 69-73. A number of patents have also described catalytic dewaxing processes. It is known that high pour point oils may be catalytically dewaxed to lower pour point oils over ZSM-5 type zeolite catalysts which selectively crack long chain normal paraffins, slightly branched isoparaffins and long chain cycloparaffins. For example, U.S. Pat. No. Re. 28,398 describes a process for catalytic dewaxing with a catalyst comprising a zeolite of the ZSM-5 type and a hydrogenation/dehydrogenation component. A process for hydrodewaxing a gas oil with a ZSM-5 type catalyst is also described in U.S. Pat. No. 3,956,102. A Mordenite catalyst containing a Group VI or Group VIII metal may be used to dewax a distillate from a waxy crude, as described in U.S. Pat. No. 4,100,056. U.S. Pat. No. 3,755,138 describes a process for mild solvent dewaxing to remove high quality wax from a lube stock, which is then catalytically dewaxed to specification pour point. U.S. Pat. No. 4,222,855 to Pelrine et al discloses dewaxing over a ZSM-23 or a ZSM-35 type catalyst.
Catalytic dewaxing processes may be followed by other processing steps such as hydrodesulfurization and denitrogenation in order to improve the quality of the product. For example, U.S. Pat. No. 3,668,113 describes a catalytic dewaxing process employing a Mordenite dewaxing catalyst which is followed by a catalytic hydrodesulfurization step over an alumina-based catalyst. U.S. Pat. No. 4,400,265 describes a catalytic dewaxing/hydrodewaxing process using a ZSM-5 type catalyst wherein gas oil is catalytically dewaxed followed by hydrodesulfurization in a cascade system.
In catalytic dewaxing processes using shape-selective catalysts, such as ZSM-5 type catalysts, the waxy components, particularly the n-paraffins, are cracked by the zeolite into lighter products including paraffins, olefins and aromatics, some of which may remain in the lube oil boiling range. Olefinic products are unstable to oxidation and must be removed. They may be removed by treatments such as hydrofinishing which uses catalysts to saturate the olefins and improve the oxidation stability of the oil. The hydrofinishing catalysts generally used are mild hydrogenation catalysts, such as a CoMo/Al.sub.2 O.sub.3 type. The color of the oil may also be improved in this hydrofinishing.
U.S. Pat. No. 4,428,819 to Shu et al discloses a process for hydrofinishing a catalytically dewaxed oil in which the residual wax content of the dewaxed oil is isomerized over a hydroisomerization catalyst. Typically, heavier lube fractions (greater than 600.degree. F. b.p.) contain waxy components comprising normal paraffins, branched paraffins and cyclo paraffins. When a shape-selective catalyst, such as HZSM-5, is used to dewax these feeds, the normal paraffins are cracked more selectively than the branched paraffins and cycloparaffins. HZSM-5 is a ZSM-5 type catalyst with only hydrogen attached to its active sites, rather than metals.
In the hydroprocessing of liquid petroleum feedstocks, and particularly so-called heavy feedstocks, it is a basic purpose of the process of catalytic dewaxing to remove lighter conversion products from the liquid petroleum flow while concurrently providing a hydrogenative environment for catalytic conversion, which is particularly utilized in the case of the processing of highly waxy feedstocks. Frequently lighter products, which are obtained from cracking and/or hydrocracking reactions, compete with the heavier feed molecules for access to the cracking sites in zeolites or silica-alumina cracking catalysts which are employed in the implementation of the catalytic dewaxing process. Inasmuch as these lighter products diffuse more rapidly into the catalyst than the larger feed molecules, they have a tendency to retard the rate of conversion of the heavier molecules. Moreover, the lighter products also tend to be either more difficult to crack, such as low molecular weight paraffins, or easier to polymerize, such as low molecular weight olefins. They also possess a tendency to coke more readily than their heavier counterparts, so as to thereby retard the conversion of the heavier molecules to an even greater extent.
This competition between the light and heavy liquid petroleum molecules is rendered particularly critical when there is employed a catalyst which essentially constitutes a shape-selective zeolite; for example, a zeolite exemplified by ZSM-5 for the dewaxing of liquid petroleum or lube stocks. Processes in reactors which utilize aluminosilicate zeolite catalysts, such as ZSM-5 or other zeolites having smaller pore openings, are disclosed in U.S. Pat. No. 4,222,855 to Pelrine et al and in U.S. Reissue Pat. No. Re. 28,398 to N. Y. Chen. Although the utilization of different types of hydroprocessing reactors to implement catalytic dewaxing processes is broadly disclosed in the prior art, as exemplified by the above-mentioned U.S. patents, broadly referring to stirring tank-type reactors or trickle bed reactors, there is an obvious need in the technology to more precisely define specific catalytic dewaxing process designs.
Current technology, as mentioned above, for dewaxing petroleum feedstocks having elevated pour points, involves the use of trickle beds whereby gas (primarily hydrogen) and the feedstock concurrently flow downwardly over a bed of solid catalyst. The three-phase trickle bed concept makes use of an intimate mixing between gas and liquid phases while in contact with the catalyst in order to facilitate dewaxing. Peformance level of the process is gauged by the length of time during which the process is producing products which meet specifications, as well as the minimum temperature required to obtain acceptable products.
U.S. Pat. No. 4,357,232 to Holland et al discloses a dewaxing process which operates at a temperature not to exceed 675.degree. to 700.degree. F., and which pretreats a dewaxing feedstock in a zeolite sorbent bed, which is a type of guard bed, prior to dewaxing.
U.S. Pat. No. 4,247,388 to Banta et al discloses treatment of zeolites to reduce an initially high alpha activity to within a range of 55 to 150 alpha prior to use as catalysts in a hydrodewaxing operation for distillate dewaxing. However, Banta et al is concerned with a steady state equilibrium (line out) distillate dewaxing process. They teach a process for decreasing steady state equilibrium temperature. In their process, as shown by their data, the catalyst deactivates from a start-of-cycle temperature of about 550.degree. F. in a few days to a steady state equilibrium temperature in excess of 700.degree. F.
Distillate dewaxing differs from lube oil dewaxing. Distillate dewaxing may operate at high temperature in excess of 700.degree. F. To produce high quality lube stock, it is necessary to restrict operating temperature to below 675.degree. F. to prevent undesirable reactions resulting in an unstable product. These reactions are less undesirable in distillate dewaxing because the product is used as fuel.
It is particularly difficult to produce lube oils by dewaxing heavy stocks comprising at least 90 wt % of compounds having a boiling point greater than 850.degree. F., such as bright stocks. Following prior art teachings, as shown by present FIG. 7, results in dewaxing bright stocks to 20.degree. F. pour point over short cycles of 15 to 30 days. The catalyst does not reach a steady state temperature before it reaches the end-of-cycle temperature.
In contrast to bright stocks, the dewaxing of light neutrals by the prior art does not have such a high steady state temperature. Therefore dewaxing of light neutrals occurs over relatively longer operating cycles, as seen by present FIG. 8.
The dewaxing summarized in FIGS. 7 and 8 occurred by contacting the respective feedstocks with a fresh alumina bound Ni-ZSM-5 catalyst having an alpha value of 108 based on zeolite. Reactor temperatures are adjusted to 20.degree. F. pour point within 1.degree. F.-2.degree. F. pour point.
Typically, a dewaxing reactor for lube oil dewaxing is operated at a start-of-cycle temperature of about 540.degree. to about 580.degree. F., usually 560.degree. F., and the operating temperature is increased by about 2.0.degree. to about 10.0.degree. F. per day, depending on feed, catalyst and space velocity, to compensate for decreasing catalyst activity and produce a lube of predetermined pour point. As seen in FIG. 8, the operating temperature may be increased less than 2.degree. F. per day after an equilibrium temperature is reached for a light neutral feedstock. Temperature is increased to an end-of-cycle temperature between about 655.degree. and about 695.degree. F., usually about 675.degree. F. At the end of a cycle the reactor is shut down and the catalyst regenerated by contact with hydrogen and, if necessary, by contact with oxygen.
It would be desirable to dewax heavy stocks to produce lube oils over extended cycle lengths. It would also be desirable to operate a dewaxing reactor at conditions which allow simple, effective H.sub.2 regeneration of catalyst without a loss of catalyst activity and allow long catalyst life, thus reducing the frequency of regeneration. Furthermore, it would be desirable to provide methods to facilitate the removal of lighter by-products during catalytic dewaxing.